The invention relates to photopolymer formulations based on a polymeric network as matrix and at least one photopolymerizable monomer dissolved therein and to a process for the production of holographic media from such photopolymers and to the use thereof. Prior to exposure to light, the photopolymer formulation has, as a measure of the crosslinking density, a particular average molecular weight MC of the segments bridging two polymer strands or a particular ratio Q of this crosslinking density to the molar mass MMo of the dissolved writing monomer, expressed as Q=MC/MMo.
Photopolymers are materials which can be exposed by means of the superposition of two coherent light sources. A three-dimensional structure forms in the photopolymers and can generally be written in the material as a result of a regional change in the refractive index. Such structures are referred to as holograms, which can also be described as diffractive optical elements. Which optical functions such a hologram forms depends on the specific exposure.
For the use of photopolymers as carriers of holograms for optical applications in the visible (λ=400-800 nm) and in the near UV range (λ=300-400 nm), colourless materials having a high diffraction effect are required as a rule after the exposure. Since the beginning of holography, silver halide films have been used for this purpose, in particular those having a high resolution. Dichromate gelatin (DCG), dichromate salt-containing gelatin films or mixed forms of silver halide and DCG are also used. Both materials require a chemical aftertreatment for the formation of a hologram, which, for industrial processes, gives rise to additional costs and necessitates the handling of chemical developer solutions. In addition, wet chemical processes result in swelling and subsequently shrinkage of the film, which can lead to colour shifts in the holograms, which is undesired.
U.S. Pat. No. 4,959,284 (Dupont) describes photopolymers which, inter alia, consist of a thermoplastic soluble in organic solvents, such as polyvinyl acetate, cellulose acetobutyrate or polymethyl methacrylate-styrene copolymers, a photoinitiator and at least one vinylcyclopropane. In addition, EP352774A1 (Dupont) describes monomers containing other vinyl groups, such as N-vinylpyrrolidone, phenoxyethyl acrylate and acrylates of triols, such as trimethylolpropane (TMPTA) and ethoxylated trimethylolpropane (TMPEOTA) or other acrylates or acrylamides. It is known in the industry that such photopolymers give usable holograms only after a relatively long thermal treatment. O'Neill et al. (Applied Optics, Vol. 41, No. 5, page 845 et seq., 2002), in their review article, discuss not only the abovementioned materials but also photopolymers which are obtainable from thermoplastics and acrylamide. In addition to the disadvantageous toxicological profile of acrylamide, such products do not give light holograms.
Holographically active materials into which it is possible to incorporate dyes which change their photosensitivity under the influence of light (Luo et al, Optics Express, Vol. 13, No. 8, 2005, page 3123) are also known. Similarly, Bieringer (Springer Series in Optical Sciences (2000), 76, pages 209-228.) describes so-called photoaddressable polymers which likewise polymer-bound dyes which can be isomerized under the influence of light. In both classes of substances, holograms can be incorporated by exposure and these materials can be used for holographic data storage. However, these products are of course highly coloured and hence not suitable for the applications described above.
More recently, photopolymers which are contained not from thermoplastics but from crosslinked polymers were also described: thus US 020070077498 (Fuji) describes 2,4,6-tribromophenyl acrylate which is dissolved in a polyurethane matrix. U.S. Pat. No. 6,103,454 (InPhase) likewise describes a polyurethane matrix having polymerizable components, such as 4-chlorophenyl acrylate, 4-bromostryrene and vinylnaphthalene. These formulations, too, were developed for holographic data storage, a holographic application in which many, but also very weak, holograms readable using electronic detectors are written and read. Common to them is the fact that the highly refracting photopolymerizable monomers are present in solution in a matrix having a low refractive index. For optical applications in the entire visible (λ=400-800 nm) and the near UV range (λ=300-400 nm), such formulations are likewise not suitable.
It was an object of the present invention to develop photopolymers for the applications as holographic media which can be processed without thermal or wet chemical aftertreatment and with which colourless holograms having a high diffraction efficiency and great brightness can be produced after exposure.
In addition to the physical properties, however, the processability and compatibility with other components are also important. Thus, organic materials which are obtained by photopolymerization, generally as homo- or copolymers of highly refracting monomers, play an important role, for example for the production of optical components, such as lenses, prisms and optical coatings (U.S. Pat. No. 5,916,987) or for the production of a contrast in holographic materials (U.S. Pat. No. 6,780,546). For such and similar applications, there is a need to be able to adjust the refractive index in a targeted manner, for example by admixing components having a high or low refractive index, and to be able to vary said refractive index over ranges. This can lead to photopolymers in which highly refracting photopolymerizable monomers are dissolved in matrices having a low refractive index or conversely photopolymerizable monomers having a low refractive index are present in solution in the highly refracting matrices.
For the abovementioned fields of use, polymers of olefinically unsaturated compounds, such as, preferably, (meth)acrylates, are typically employed. In order to achieve a refractive index of 1.5 or higher, halogen-substituted aromatic (meth)acrylates or special alkyl methacrylates described in U.S. Pat. No. 6,794,471 can be used. In particular the latter are disadvantageous owing to their complicated preparation.
The suitability of substituted phenyl isocyanate-based urethane acrylates for the preparation of corresponding polymers was described by Bowman (Polymer 2005, 46, 4735-4742).
The non-prior-published WO application PCT/EP2008/002464 discloses (meth)acrylates having a refractive index at λ=532 nm of at least 1.5, which are suitable for production of optical data media, in particular those for holographic storage methods, and are based on industrially available raw materials. In this context, phenyl isocyanate-based compounds are also known, these always being based on unsubstituted phenyl rings on the isocyanate side.
In photopolymer formulations, highly refracting acrylates play a decisive role as a contrast-imparting component (U.S. Pat. No. 6,780,546). The interference field of signal light beam and reference light beam (in the simplest case two plane waves) is formed by the local photopolymerization at locations of high intensity in the interference field by the highly refracting acrylates in a refractive index grating which contains all information of the signal (the hologram). By illuminating the hologram only with the reference light beam, the signal can then be reconstructed again. The maximum strength of the signal thus reconstructed in relation to the strength of the incident reference light is referred to as Diffraction Efficiency, DE below. In the simplest case of a hologram which forms from the overlap of two plane waves, the DE is obtained from the quotient of the intensity of the light diffracted on reconstruction and the sum of the intensities of incident reference light and diffracted light. The higher the DE, the more efficient is a hologram with respect to the necessary quantity of light of the reference light which is necessary to make the signal visible with a fixed brightness. Highly refracting acrylates are capable of producing refractive index gratings having a high amplitude Δn between regions with the lowest refractive index and regions with the highest refractive index and thus permitting holograms with high DE in photopolymer formulations. (The refractive index contrast □n which results on writing a volume hologram by means of the overlap of two plane waves is obtained from the following refractive index variation n(x)=n0+Δn·cos(K·x), where K represents the magnitude of the grating vector which points in the direction of the x-axis and n0 represents the mean refractive index. See, for example, Hariharan Optical Holography, Principles, Techniques and Applications, Cambridge University Press, 1991 page 44.)
U.S. Pat. No. 6,939,648B describes optical articles obtained from photopolymer formulations which are based on a crosslinked polyurethane matrix and have a modulus of elasticity E of at least 0.1 MPa, the thickness of the photopolymer layer being greater than 200 μm. It is disclosed that, the greater the modulus of elasticity, the more preferred the photopolymer formulation is said to be. It is not specified how the modulus of elasticity is measured and how it is to be understood in relation to the topology and dynamic properties of the matrix polymer strands, i.e. whether it characterizes the photopolymer state crosslinked in a rubber-like manner or the photopolymer state solidified in a glassy manner. The relationship between crosslinking density, writing monomer molecular weight and the holographic performance in the case of individual strong holograms is not disclosed, in particular not for reflection holograms. On the contrary, the preferred direction described in the abovementioned application leads to higher modulus of elasticity and, when writing individual strong holograms, to a deterioration in the holographic performance, as can be seen from the examples disclosed here.
A known procedure for optimizing the performance of photopolymers in holographic applications is therefore to increase the difference between the refractive indices of the matrix polymer and of the writing monomer dissolved therein, for example by dissolving highly refracting writing monomers in matrices having a low refractive index or using writing monomers having a low refractive index in highly refracting matrices.
If the matrix is formed as a polymeric network, the mechanical, optical, thermal and thermodynamic properties of the photopolymer can be established in a targeted manner within wide limits by the choice of the network-building repeating units and the functionalities thereof. The prior art described above does not disclose whether and to what extent the crosslinking density of such photopolymers can decisively influence the performance in holographic media.
It has now surprisingly been found that photopolymer formulations based on a matrix which represents a polymeric network and at least one photopolymerizable monomer dissolved therein produce refractive index gratings having high amplitude (Δn) between regions with the lowest refractive index and regions with the highest refractive index in holographic media in particular when a low crosslinking density of the photopolymer formulation is present prior to the exposure to light. Such photopolymer formulations are therefore particularly suitable for producing bright, visual holograms having high diffraction efficiency in holographic media as described above. Visual holograms comprise all holograms which can be recorded by methods known to the person skilled in the art, including, inter alia, in-line (Gabor) holograms, off-axis holograms, full-aperture transfer holograms, white light transmission holograms (“rainbow holograms”), Denisyuk holograms, off-axis reflection holograms, edge-lit holograms and holographic stereograms; reflection holograms, Denisyuk holograms and transmission holograms are preferred.